Lubricants: mineral, synthetic or biodegradable?

Mineral oil-based lubricants have long been the fluid of choice for the hydraulics industry, but is this changing with the ongoing development of synthetic and biodegradable solutions?

ProcessOnline spoke with Jeanette Wyatt of Fuchs Australia to find out what new technology is available in lubricants and the advantages and disadvantages of moving away from a mineral oil-based product.

She explains that there are many characteristics of a lubricant that need to be considered when matching it to the application at hand.

Is petroleum-based oil still the most popular choice for industry?

Yes. Mineral oil-based products remain the most cost-effective option for use in systems that are operating in 'normal' environments. For example, those that are not operating at extremes of temperature such as arctic conditions or extreme heat.

What alternatives are now becoming popular?

The use of biodegradable oils based on either vegetable oils such as canola or synthetic esters are becoming popular due to the growing emphasis on protecting the environment.

What are the important characteristics to look for when selecting a hydraulic fluid?

Correct selection of hydraulic fluid depends on the application, such as the working temperature range, design of the hydraulic system, type of pump, working pressure, required fluid life and environmental considerations.

All of these factors determine the selection of fluid with correct viscosity, and chemical composition. Ideally, a hydraulic fluid should have the following properties:

• Incompressible
• Have good anti-wear properties
• Resistant to oxidation
• Inactive to system materials such as elastomers and metals
• Good air separation
• Low foaming
• High heat transfer rate
• Good water release
• Good shear stability
• Low evaporation

Viscosity should be as low as possible to ensure fast response of hydraulics and adequate lubrication on start-up, yet have a suitable minimum viscosity to reduce leakages and guarantee lubrication of the pump and other moving parts.

Can you compare the types of fluids available and the characteristics they offer?

There are various ways to classify hydraulic fluids and numerous systems abound (ISO, DIN, 7th Luxembourg report etc). The most common is by base oil type, and then it may be further broken down into various sub-classifications. Fire-resistant fluids and food-grade fluids have their own classification systems.

There are four basic fluid types - mineral oil, biodegradable (may also be synthetic and/or fire resistant), synthetic (may also be biodegradable and/or fire resistant) and water containing (water-glycol).

Mineral oil products offer good, all-round performance. The main limitation is relatively poor oxidation resistance when compared to synthetic fluids.

Most applications are adequately covered by use of mineral oil products. The exception to this is extremes in operating temperatures such as extremely high or extremely low ambient temperatures, where biodegradability or fire resistance is required, or if there is a possibility of contact with food.

Biodegradable fluids such as those based on vegetable or synthetic esters are recommended where there is a risk of spillage into the environment. Depending on the base oil type, some biodegradable fluids also offer fire resistance.

These fluids offer excellent performance; however, the main downside is that they rapidly become acidic if water is allowed to enter the system, resulting in attack on metallic components.

There are a number of different types of synthetic fluids available, all with differing advantages and disadvantages. In general, they offer better oxidation resistance to mineral oil products, but can be more prone to attack by water (as in the case of esters).

The biggest disadvantage is a higher cost relative to mineral oil-based fluids. Therefore, synthetics are generally only used where the operating environment or system design requires it, such as at extremes of temperature or in environments where fire resistance is required.

The main advantage with water-containing products is their fire resistance and the use of these types of fluids is mandatory in some industries. Disadvantages include an incompatibility with certain metal and common elastomers. Therefore, when changing to these types of fluids a full system check is required.

Systems running these fluids also lose water through evaporation, which affects fire-resistance properties and viscosity (with resultant effects on hydraulic response etc). Diligence is required when replenishing water content, as corrosion protection offered by the fluid may be compromised.

What are the common problems associated with hydraulic oils?

Oxidation of the oil is generally the biggest issue. As oils oxidise, they increase in viscosity, leading to poor heat transfer, sluggish hydraulic response, reduced protection of pump and moving parts on start-up.

Additionally, they become acidic, leading to an attack on metallic components in the system. Badly oxidised oils also directly cause the build-up of sludge and varnish in the system.

How important are oil analysis systems in detecting or eliminating these problems?

Regular monitoring of the oil's properties (such as viscosity, extent of oxidation, additive levels) and the monitoring of system wear metals are important in extending or maximising the life of the fluid. These play a key role in predicting accelerated component wear and preventing breakdowns.

An appropriate condition monitoring and predictive maintenance schedule in the long term ensures more reliable system operation and reduces overall costs.

What is the trend in using such systems?

Use of oil analysis seems to be growing, yet many users do not utilise these effectively by not keeping accurate records of top-up volumes or frequencies and not sampling frequently enough to develop baseline data for the correct interpretation of results. It must be remembered that a single result does not constitute condition monitoring.

Don't miss next month's ProcessOnline newsletter, where we investigate current trends in the use of oil analysis systems.